Many articles of manufacture contain one or more brazed joints. In many instances the braze alloy simply has to be metallurgically compatible with the metal members being joined and provide requisite mechanical strength. But in some applications a brazed joint requires additional properties such as resistance to environmental corrosion. This additional requirement may add dramatically to the cost of the braze material and the finished product. An example of such a brazing application is found in water-cooled brazed copper assemblies such as resistance welding transformers.
Electrical resistance welding is widely used in the manufacture of automotive vehicle bodies and other articles of manufacture. Such welding operations require electrical transformers to provide electrical currents of 5,000 to 50,000 amperes at a voltage suitable for driving the current through welding tools and the workpiece(s). Several thousand spot welds are formed on each car body and a manufacturing plant may have a thousand weld transformers. The secondary windings of welding transformers carry large currents and are formed of single-turn or double-turn, thick-walled copper tubes with copper pads (castings) brazed to each end of the tube. The wrought tubes and cast pads are typically made from high conductivity electronic grade copper. The end pads have openings for flow of cooling water at 50 psi in the tubular windings and may further provide for electrical connections for delivery of current from the transformer winding. The secondary winding may be embedded in non-conductive epoxy resin.
The tubes and pads are joined during assembly with a torch brazing process and filler metals from the copper-silver-phosphorus braze metal family. These alloys have been used extensively in welding transformers. They are economical and can be self-fluxing when used to braze copper. The copper phosphide family of braze metals are also used for joining copper and high-copper alloys in other applications, particularly copper-to-copper tube including copper piping in cooling systems.
Although similar brazed transformers have enjoyed an excellent reputation for durability, the occurrence of failed transformers from water leaks has been observed often enough to warrant investigation. Examinations of failed joints revealed preferential attack of copper in and adjacent to Cu—Ag—P braze filler metals. Evidence indicated that the corrosion was initiated by the presence of sulfide-containing ions in the cooling water which act aggressively on copper phosphide in the braze region to corrode the joint by galvanic action.
A proposed corrective action is to use a phosphorus-free, silver-copper-zinc-tin braze filler metal. The high silver content and lack of phosphorus of this braze filler metal makes it less susceptible to corrosion from contact with water but much more expensive than the Cu3P-containing conventional braze filler. Other solutions to brazing applications involving corrosion-threatened surfaces are needed.